Implications of Heat Treatment and Systemic Delivery of Foliar-Applied Oxytetracycline on Citrus Physiological Management and Therapy Delivery.

Christopher Vincent,Myrtho Pierre,Nian Wang,Jinyun Li

doi:10.3389/fpls.2019.00041

Christopher Vincent, Myrtho Pierre + Show 2 more

Open Access

https://doi.org/10.3389/fpls.2019.00041

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Abstract

Huanglongbing is an economically devastating disease of citrus in Florida and around the world. This study was undertaken to assess two grower-used therapies, heat treatment, and foliar anti-bacterial application. Specifically, there was an industry claim that heat treatment improved subsequent systemic uptake of foliar-applied anti-bacterial compounds. We hypothesized that new vegetative growth induced by heat treatment could lead to increased foliar delivery because of a greater number of new leaves in which cuticles would be more permeable. The study included two factors (1) heat treatment (with or without) and (2) pruning, in which all new leaves, all mature leaves, or no leaves were removed. A commercial formulation of oxytetracycline (OTC) was applied to plants with a non-ionic penetrant surfactant, but one branch on each tree was covered to assess direct versus systemic delivery. The study was repeated twice, destructively assessing whole-plant leaf area and dry weights, as well as OTC content in directly applied and covered leaves. Heat treatment and defoliation treatments reduced growth, but did not affect systemic delivery of OTC. OTC was detected in nearly all covered leaf samples in both repetitions, though at lower concentrations than in directly applied leaves. We conclude that neither heat treatment nor leaf age strongly affect systemic OTC delivery. Implications of this study for leaf age effects on foliar delivery and for phloem delivery of foreign compounds through foliar application are discussed.

Highlights

Huanglongbing disease (“citrus greening”; HLB) is one of the most economically damaging diseases of perennial crop plants. It is caused by the persistent infection of a phloem-limited bacterium Candidatus Liberibacter asiaticus (Las), which leads to hyperaccumulation of carbohydrates in leaves, and starvation of sink tissues (Cimò et al, 2013), resulting in reduced overall growth, leaf senescence, loss of root length and mass density, reduced fruit production and increased pre-harvest fruit drop
heat treatment (Heat) treatments induced a noticeable defoliation, as is presented in the plant growth section, and one tree in the Heat-Mature Leaves treatment died in each trial
By the time gas exchange and chlorophyll fluorescence were measured 3 weeks later, there were no significant effects on chlorophyll fluorescence, photosynthesis, respiration, or stomatal conductance of the leaves that remained

Summary

Introduction

Huanglongbing disease (“citrus greening”; HLB) is one of the most economically damaging diseases of perennial crop plants It is caused by the persistent infection of a phloem-limited bacterium Candidatus Liberibacter asiaticus (Las), which leads to hyperaccumulation of carbohydrates in leaves, and starvation of sink tissues (Cimò et al, 2013), resulting in reduced overall growth, leaf senescence, loss of root length and mass density, reduced fruit production and increased pre-harvest fruit drop Because of the economic impacts of HLB, therapies to reduce populations of Las within infected plants in the field have been an area of research as well as grower practice These therapies include steam or hot-water generated heat treatments and the use of anti-bacterial compounds (Doud et al, 2014; Hu et al, 2017)

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